S 1892 
9py 1 



DEPARTMENT OF COMMERCE AND LABOR 

BUREAU OF STANDARDS 

S. W. STRATTON, Director 



THE DETERMINATION OF TOTAL SUL- 
PHUR IN INDIA RUBBER 



BY 

C. E. WATERS. Associate Chemist 

and 

J. B. TUTTLE, Assistant Chemist 

Bureau of Standards 



[JULY 19, 1911] 



REPRINT NO. 174 
(FROM BULLETIN OF THE BUREAU OF STANDARDS, VOL. 8, NO. 3) 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1912 



DEPARTMENT OF COMMERCE AND LABOR 

BUREAU OF STANDARDS 

S. W. STRATTON, Director 



THE DETERMINATION OF TOTAL SUL- 
PHUR IN INDIA RUBBER 



BY 

C. E. WATERS, Associate Chemist 

and 

J. B. TUTTLE, Assistant Chemist 

Bureau of Standards 



[JULY 19, 1911] 

REPRINT NO. 174 
(FROM BULLETIN OF THE BUREAU OF STANDARDS, VOL. 8, NO. 3) 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1912 



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THE DETERMINATION OF TOTAL SULPHUR IN INDIA 

RUBBER 



By C. E. Waters and J. B. Tuttle 



What may be still described as the usual method for the deter- 
mination of total sulphur in india rubber is the one first pub- 
lished by Henriques.^ The details of this method are too well 
known to require description here. In more recent years other 
methods have been advocated. Alexander ^ used sodium peroxide 
to decompose the nitrogen peroxide addition-product of rubber. 
In the same year Ksch ' recommended the use of Eschka's mix- 
ture and procedure for the determination of sulphur m coal. He 
also stated that the sodium-peroxide method gives good results. 
Wagner * published a slight modification of the method of Hen- 
riques stating that much sulphur is lost by volatilization. He. 
therefore made the nitric-acid solution alkaline with sodium 
hydroxide, transferred to a nickel crucible, added sodium carbo- 
nate and then evaporated to dryness. The oxidation was earned 
to completion by heating in an air-bath. 

Pontio^ fused with manganese peroxide and a mixture ot 
sodium and potassium carbonates. The results were about o. i per 
cent lower than by the method of Henriques. For the free sul- 
phur « he extracted with absolute alcohol, distilled off the solvent, 
oxidized with alkaline hydrogen peroxide, evaporated to dryness 
and fused in a silver crucible. ^__ 

1 Z. angew. Chemie, 12, p. 902; 1899. 

2 Gummi-Ztg., 18, p. 729; Z. angew. Chemie, 17, p. 1799; 1904. 

3 Chem.-Ztg., 28, p. 200; 1904. 

* Gummi-Ztg., 21, p. 552: Chem. Abstr.. 1. p. 1327; 1907. 

6 Caoutchouc et Gutta-Percha. 6, p. 2751; Chem. Techn. Rep., 1909; 372. 

6 Caoutchouc et Gutta-Percha. 5, p. 2194; Chem. Abstr.. 2, p. 3412; 1908. 

445 



446 Bulletin of the Bureau of Standards I'^oi. s. No. $ 

A distinct departure from the usual methods is due to Hin- 
richsen/ who oxidizes electrolytically in the presence of concen- 
trated or fuming nitric acid. 

Finally, Hiibener ^ devised a method intended to exclude insolu- 
ble mineral sulphates. The sample is boiled in a flask with con- 
centrated nitric acid for some time, most of the acid is evaporated 
off on the steam-bath, and the oxidation completed by means of 
bromine and water. 

One of the present writers, having frequent occasion to deter- 
mine total sulphur in rubber, over a year ago made a number of 
comparative tests of different variations of the method of Hen- 
riques. The results obtained with two samples of rubber are 
given below (I-V). In all cases, 0.50 g of rubber was taken. 
All fusions were made over a flame of gasoline-air gas. The results 
are given as percentages of sulphur. All reagents were tested, and 
no determinations have been omitted. 

I. Warmed 2}4 hours in covered crucible with 25 cc cone. HNO3, 
allowed to stand 36 hours, evaporated nearly to dryness, added 
NaaCOa.KNOs mixture and fused as usual. 

Sample i i 2 2 

Sulphur 3.39 3.44 3.26 3.22 

II. Added HNO3 and i cc Br, let stand 36 hours without pre- 
liminary heating, evaporated, etc., as usual. 

Sample i i 2 2 

Sulphur 3.40 3.47 3.39 3.27 

III. The same as II, but allowed to stand only i hour, headed 
with cover for 2 hours, evaporated and fused as usual. 

Sample i i 2 2 

Sulphur 3.31 3.35 3.04 3.09 

IV. Only HNO3 added, digested at once on the steam-bath for 
2 hours, evaporated and fused. 

Sample i i 2 2 

Sulphur 3.17 3.43 3.06 2.93 

' Chem.-Ztg., 33, p. 735; 1909. 

' Gummi-Ztg., 24, pp. 213-214; Analyst, 35, pp. 266-267; 1910- 



Waurs-\ Determination of Sulphur in Rubber 447 

Tuttle J 

V Treated with i cc Br and 5 cc H^O, let stand over night with- 
out heating, next morning evaporated off the H3O, added HNO3, 
digested, evaporated and fused. 

Sample i i 2 2 

Sulphur 3-71 3-65 3-37 3-38 
In an attempt to obtain satisfactory resuhs without fusion 
some determinations were made some months later without 
a knowledge of Htibener's paper. Half-gram portions of a sample 
of medium hard rubber were digested with nitric acid m flasks 
covered with watch glasses. In some cases bromine was added 
after the digestion with acid and, after standing half an hour, 
water was added and the flasks heated on the steam-bath. Finally 
the volume was brought to about 175 cc, the solution heated 
mtered, and a little sodium hydroxide added to the filtrate and 
wash-water. This was then evaporated to dryness, adding a little 
hydrochloric acid toward the end, taken up with very dilute 
hydrochloric acid, filtered and barium sulphate precipitated as 
usual. The results follow : 

VI. Treated with HNO3 alone. 

Sulphur 7-76 7-51 7-68 7-96 

VII. Treated with HNO3, followed by Br. 

Sulphur 7-62 7-51 7-93 7-87 7-76 
All the precipitates obtained under VI and VII contained much 

After the method of Hubener was called to our attention some 
determinations were made on a sample of hard rubber contammg 

no barium. 

VIII. Hiibener's method. 

Sulphur 4-79 3-9i 5-23 4-02 4-3i 4-i3 
It is evident that widely different amounts of sulphur must 
have been retained in the insoluble residue m the form of lead 

sulphate. 

IX. Total sulphur by method of Henriques. 

Sulphur 8.65 8.70 

X. Treated with HNO3, followed by Br and H3O and fused as 

''"'""'' Sulphur 8.63 8.62 8.77 8.80 8.72 8.80 



448 Bulletin of the Bureau of Standards [Voi. s. no. 3 

It has recently been claimed by van't Kruys ^ that when an 
excess of calcium chloride over the amount of sulphuric acid is 
present, only calcium sulphate is carried down with the barium 
sulphate, and the calcium salt can be converted into barium 
sulphate by digestion with strong hydrochloric acid, or aqua 
regia, and barium chloride. Several determinations were made 
to test this suggestion. 

XI. Preliminary treatment as under X, subsequent treatment 
as suggested by van't Kruys. 

Sulphur 8.73 8.76 8.52 8.46 8.74 8.78 8.75 8.77 
At this point, joint analyses of a fairly large sample of rubber 
were carried out by the present writers. 

XII. Hiibener's method. The sulphur in the insoluble residue 
was determined by fusion with soda-saltpeter mixture, extracting 
the melt with water, etc., as usual: 

S in original filtrate 0.91 0.91 0.68 0.79 0.82 0.99 1.52 1.15 1.24 
S in insoluble residue 2.35 2.43 2.54 2.69 2.53 2.36 1.95 2.31 2.21 
Total sulphur found 3.26 3.34 3.22 3.48 3.35 3.35 3.47 3.46 3.45 
All the precipitates of barium sulphate from the original 
filtrates were found to contain lead when tested with dilute 
ammonium sulphide. 

XIII. Treated with HNO3, allowed to stand over night, the acid 
driven off on the steam -bath, i cc Br and 10 cc HjO added; then 
the H2O and excess of Br driven off by heating. The residue was 
mixed with soda and saltpeter and fused as usual. 

Sulphur 3.41 3.21 

XIV. The same as XIII, but the HNO3 not driven off before 
adding Br. 

Sulphur 3.60 3.63 3.58 3.63 3.57 

XV. The same as XIII, but treatment with Br omitted. 
Sulphur 3.29 3.49 3.35 3.38 3.55 3.43 3.36 3.58 3.56 

XVI. Treated first with bromine and water, allowed to stand 
over night without heating, then Br and HoO driven off on steam- 
bath, treated with HNO3, ^tc, and fused. 

Sulphur 3.45 3.47 3.48 3.53 3.49 

XVII. The same as XVI, but excess Br and H2O not driven off 
before adding HNO3. 

Sulphur 3.59 3.47 3.64 

' Zs. anal. Chem., 49. p. 393; 1910. 



Waters! Determination of Sulphur in Rubber 449 

XVIII. The method of Henriques, except that the HNO3 was 
saturated with Br. 

Sulphur 3.66 3-65 3-62 3-73 3-68 3-65 3-69 3-63 3-66 3-7i 3-62 

XIX. The same as XVIII, but followed by the treatment 
suggested by van't Kruys. 

Sulphur 3.69 3-73 3-76 3-75 
In order to obtain a definite idea of the variations caused by 
differences in the preliminary treatment and in the conditions 
under which the barium sulphate is precipitated, a very dilute 
solution of sulphuric acid was made. In each of the following 
determinations, a 25-cc portion was taken. The weights of the 
barium sulphate found were calculated as percentages of sulphur 
in 0.50 g of rubber, in order that the results might be more readily 
compared with the determinations above. 

XX. Direct precipitation with BaClj. The last two determina- 
tions were made with the addition of 2 cc of i : i HCl, the first 
six without adding HCl. 

Sulphur 3. II 3-II 3-II 3-ii 3-ii 3-ii 3-io 3.11 

XXI. Evaporated off the water from 25 cc of the dilute H2SO4, 
added the soda-saltpeter mixture and fused as usual. 

Sulphur 3.12 3.15 3-12 

XXII. Like XXI, but added CaCl^ to the solution of the melt 
before precipitating BaSO,, and treated the latter according to 

van't Kruys. 

Sulphur 3.22 3.18 3.19 

XXIII. Like XXII, but did not digest the precipitated BaSO* 
nor evaporate the filtrate to recover traces of dissolved BaSO^. 

Sulphur 3.13 3-12 3-1 1 

XXIV. Like XXI, but did not fuse. The solution was acidi- 
fied with HCl. 

Sulphur 3.17 3-19 3-i6 

XXV. Added 250 cc H^O and 10 cc cone. HCl to 25 cc dilute 
H SO added Bad,, digested two hours, poured off the super- 
natant Hquid, digested the BaSO, with i cc of 10 per cent BaCl, 
and 15 cc HCl (1:1). Diluted, filtered, evaporated the combined 
filtrates to dryness in platinum, took up with 50 cc of slightly 
acidified H,0 and collected the sUght residue on the same filter. 

Sulphur 3.12 3.10 3-1 1 



75-56 


75-55 


.2082 


.2080 


.34197 


.34164 


.1131 


.1130 


3-II 


3-11 



450 Bulletin of the Bureau of Standards [Voi. 8, No. 3 

XXVI. Exactly neutralized NaOH solution with the dilute 
H2SO4, using phenolphthalein as indicator. After each addition 
of acid, the solution was heated in a platinum dish until the pink 
color no longer reappeared. Then evaporated to dryness, ignited 
gently, and weighed the NagSOi. 

H2SO4 used (cc) 

Na^vSO^ found (g) 

BaSO^ equivalent to Na2S04 (g) 

BaSO^ equivalent to 25 cc H2SO4 (g) 

Sulphur (calc. on 0.5 g rubber) 

XXVII. The same as XXVI, but used NaHCOg instead of NaOH. 

H2SO4 used (cc) 45 -oi 43 -4^ 

Na2S04 found (g) 

BaSO^ equivalent to Na2S04 (g) 

BaSO^ equivalent to 25 cc H2SO4 (g) 

Sulphur (calc. on 0.5 g rubber) 

In order to test the completeness of the oxidation of sulphur by 
means of the nitric acid-bromine mixture, the following determina- 
tions were carried out. 

XXVIII. Powdered sulphur crystals, digested in the cold with 
20 cc HNO3 and an excess of Br. Finally added 20 cc H2O and 
heated on the steam-bath for about two hours. Then evaporated 
nearly to dryness, took up with water, and precipitated with 
BaClj. 

Sulphur taken (g) 0.0483 0.0395 0.0561 
Sulphur found (g) .0481 -0399 -0563 

XXIX. Powdered sulphur crystals treated at the same time 
as some of the samples of rubber. The exact methods are referred 
to in the table, the Roman numerals indicating the method 
employed. 



.1240 


.1194 


.20367 


.19612 


.1131 


.1128 


3-II 


3.10 



Method 


I 


II III IV 


V 


vSulphur taken (g) 


0.0528 


0.0619 0.0494 0.0646 


0.041 1 


Sulphur found (g) 


.0485 


.0590 .0479 .0595 


.0414 


Sulphur, per cent 


91.84 


95-33 96.91 92.13 


100.67 



Yuia/] Determination of Sulphur in Rubber 451 

In tne determinations by methods I to IV, part of the sulphur 
was not attacked by the nitric acid, nor by the sodium carbonate 
added before making the fusion. Part, at least, of this unattacked 
sulphur was seen to burn when the fusion was made. 

As stated above (XII), the barium sulphate precipitates repre- 
senting soluble sulphates, etc., in the Hiibener method, were 
found to contain lead. Lead sulphate dissolves shghtly, and is, 
besides, partially decomposed by water, hydrobromic and nitric 
acids, etc.^" In order to get an idea of the amount of barium 
sulphate to be expected to result from the decomposition and 
solution of lead sulphate under the conditions of the Hiibener 
method, some determinations were made. Lead sulphate was 
first prepared by precipitation from a hot, dilute nitric-acid 
solution of lead nitrate by means of a hot, dilute solution of 
sulphuric acid. It settled rapidly as a coarse-grained powder, 
which was washed by decantation with hot water, then in a Gooch 
crucible with hot water, followed by strong alcohol. It was then 
dried in an air-bath. 

In the first experiments it was treated with hot water, and the 
amount of barium sulphate precipitated from the filtrate was 
calculated as percentage of sulphur in 0.50 g rubber. 

XXX. Washed 0.200-gram portions of PbSOi on filters. 
Each time 250 cc hot water was used. The filtrates were slightly 
acidified with HCl, and precipitated with BaClz- 

vSulphur 0.24 0.25 

These precipitates contained only traces of lead. 

XXXI. Treated 0.200-gram portions of PbSO^ according to 
Hiibener's method, slightly modified. Treated with 13 cc cone. 
HNO3, evaporated practically to dryness on the steam-bath, added 
50 cc H2O, and 0.5 cc Br and 2 cc of dilute HNO3 (1:4). Heated, 
filtered, and washed with about 200 cc of hot water. Then pre- 
cipitated with BaCls. 

Sulphur 0.73 0.80 0.57 

These precipitates contained a little lead. 

1° Kolb: Dingl. pol. J., 209, p. 268. Ditte: Ann. Chim Phys. [5], 14, p. 190. 



452 Bulletin of the Bureau of Stmidards [Voi.s. xo.j 

From these determinations it seems quite certain that the 
larger part of the sulphur found as soluble sulphate under XII 
must have come from the solution and decomposition of lead sul- 
phate first formed when the rubber was attacked by nitric acid. 

At the suggestion of Dr. Hillebrand, four determinations were 
made of the amount of lead carried down with the barium sul- 
phate precipitated in the usual way from the aqueous extract of 
the fusion mass. In spite of the presence of a large excess of 
sodium carbonate, some lead goes into solution. The preliminary 
treatment was according to XVIII, and 2 g of rubber instead of 
0.50 g was taken each time. 

XXXII. After fusion, the melts were dissolved in water. To 
each of the first two there was added 2 g of sodium bicarbonate 
in order to decompose any alkali plumbate. The solutions were 
heated on the steam-bath, for one and one-half hours and then 
filtered from the insoluble. After' acidifying w4th hydrochloric 
acid, barium sulphate was precipitated in the usual way. 





I 


2 


3 


4 


BaSO^ found (g) 


0.5379 


0-5395 


0.5437 


0.5417 


Sulphur (per cent) 


369 


3-71 


3-73 


3-72 



The barium sulphate precipitates were then mixed with soda 
and potash and fused. The melts were dissolved in water, filtered, 
and the residues washed with hot, very dilute sodium carbonate 
solution. The residues of barium carbonate and lead oxide were 
then dissolved in dilute nitric acid and the lead precipitated 
from the cold solutions by hydrogen sulphide. After standing 
over night in stoppered flasks, the precipitates of lead sulphide 
were filtered off, washed, dissolved in nitric acid, and finally con- 
verted into sulphate by evaporating down in porcelain crucibles 
with sulphuric acid and gently igniting. 



PbSO^ found (g) 
Equivalent to BaSO^ (g) 
Corrected BaSO^ (g) 
Corrected sulphur (per cent) 



I 


2 


3 


4 


0.0086 


0.0071 


0.0040 


0.0045 


0.0066 


0.0055 


0.0031 


0.0035 


0.5359 


0.5379 


0.5428 


0.5407 


3.68 


3-69 


3-73 


3.71 



Waters 
Tuttle 



J Determination of Sulphur in Rubber. 453 



It is quite evident from these figures that ahhough notable 
quantities of lead sulphate are carried down with the barium sul- 
phate, the correction in the percentage of sulphur is negligible. 

The filtrates from the original precipitates were treated with 
hydrogen sulphide and gave slight precipitates. The alkaline 
filtrates from the barium carbonate and lead oxide were tested 
with ammonium sulphide and became brown. The next day 
there was a slight film of a dark color on the bottom of each of the 
beakers in which these solutions were tested. This was probably 
a mixture of small amounts of lead and iron sulphides. In all 
the solutions tested, as well as in the actual determinations of 
lead sulphate, greater amounts of lead were found in i and 2, 
which had been treated with bicarbonate. Apparently at the 
temperature of the steam-bath the lead bicarbonate probably 
formed was not decomposed. 

CONCLUSIONS 

Treatment of the rubber with nitric acid alone gives low results. 
(Compare XV with XVIII.) This is probably largely due to loss 
of free sulphur, since nitric acid alone does not completely oxidize 
sulphur to sulphuric acid in the length of time ordinarily taken 
for a determination. 

The Hiibener method can not be employed in the presence of 
mineral fillers which tend to form insoluble sulphates. This 
applies especially to barium carbonate and litharge. 

A comparison of XX to XXVII shows that the fusion method 
gives results very close to those obtained by direct precipitation 
and by neutralization. The van't Kruys method gives high 
results. 

The best results seem to be obtained by the use of method 
XVIII, according to which the rubber is decomposed by means of 
nitric acid saturated with bromine. 

Washington, July 19, 191 1. 



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